Developer cartridges that are detachably mounted in an image-forming device are well known in the art. One such developer cartridge disclosed in U.S. Pat. No. 6,823,160 includes a developing roller, and an input gear for receiving a driving force from the main body of the image-forming device and transmitting the driving force to the developing roller. A coupling member is also provided in the main body of the image-forming device for engaging with and rotating the input gear in the developer cartridge. This coupling member is made to engage with and retract from the input gear in association with the opening and closing of a cover on the main body of the image-forming device when mounting and removing the developer cartridge.